Downy mildew is a serious pest worldwide, in both glasshouse and open field production of lettuce plants (Lactuca sativa L.). Lettuce downy mildew is caused by the fungus Bremia lactucae, belonging to a class of relatively primitive fungi known as Oomycetes. The fungus is an obligate parasite capable of infecting a lettuce plant in any growth stage from seedling to mature plant.
Co-evolution of the plant and the pathogen has led to an arms race in which the resistance of the plant can be broken down as a consequence of the capability of the pathogen to interact with and modify alternative host targets or the same targets in a different way.
One breeding technique to slow down the fast gain of virulence from Bremia population is to associate different Bremia resistance genes (also called pyramiding or stacking). One limit of this strategy is that the different Bremia resistance genes (R-genes) are grouped in a limited number of locations. Such locations are called resistance gene clusters, In lettuce, at least 4 major resistance gene clusters are known lying on chromosome 1, chromosome 2, chromosome 4, and chromosome 8, respectively. R-genes from the same cluster can segregate as alleles or tightly linked genes. Therefore, it is often impossible to stack R-genes from the same cluster, because genes on the same cluster are in repulsion phase with each other, and are inherited like alternative alleles of the same locus. In breeding lettuce, which is a diploid crop, the opportunity of pyramiding dominant Bremia-genes on the same resistance gene cluster is therefore limited to a maximum of two genes per duster. This maximum can only be reached in heterozygous plants with a single copy for each gene. However, providing heterozygous lettuce plants to growers implies the use of hybrid varieties, Unfortunately the production of hybrid lettuce seed is considered too expensive, too risky and too complicated. Therefore in practice almost all lettuce varieties are inbred lines. In an inbred line the number of genes per cluster is limited to one, which is then present in a homozygous state with two copies for the gene.
As mentioned earlier, there is a continuous arms race going on between the plant and the pathogen and during this race R-genes are constantly broken and breeders need alternative resistance sources in order to keep producing resistant varieties. Every new combination of R-genes is therefore a valuable asset for the breeder.
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